Molten metal stirring and vacuum degassing



yi 1967 v KQC. TAYLOR 3,330,900

MOLTENMETAL STIRRING AND VACUUMDEGASSING Filed Sept. 15. 1964 42/ I r 52L 7 INVENTORS V THEODORE IRKE/V/VED) RENO/PICK C. MVLOR ATTOR/VEVJUnited rates Patent @hfice 3,330,900 Patented July 11, 1967 3,330,900MOL'IEN METAL STIRRING AND VACUUM DEGASSING Kendrick C. Taylor, Oreland,Pa, and Theodore R.

Kennedy, Burlington, N.J., assignors to Pennsalt Chemical Corporation,Philadelphia, Pa, a corporation of Pennsylvania Filed Sept. 15, 1964,Ser. No. 396,670 15 Claims. (Cl. 1328) In'general, this inventionrelates to a new and improved method and apparatus for molten metalstirring. More particularly, it is directed to the use of polyphasestirring of molten metal in a ladle to produce unidirectional stirringof molten metal therein.

The benefits of stirring molten metal for alloying vacuum, deoxidizing,degassing and the like have been known for many years. For variousreasons, it is desirable that this stirring should take place in theteeming ladle.

Since practically all ladles used in a modern steel plant areessentially a substantial steel shell with a suitable refractory liner,the application of stirring forces by electromagnetic means wasdiscouraged by the shielding action of the steel ladle shell. Further,it was generally considered hazardous and impractical to place effectiveinduction winding within the refractory liner of the ladle shell.

Further, there is a need for a molten metal stirrer which can beutilized in a modern steel mill without the necessity of redesigningequipment presently in service. The molten metal stirring apparatus mustfurther be easily transportable and capable of being utilized on varioustypes of equipment,

In view of the foregoing, it is the general object of this invention toprovide a new and improved molten metal stirrer.

A further object of this invention .is the provision of a new and bettermolten'metal stirrer which can be utilized with any teeming ladle.

Still another object of this invention is to provide a new and bettermolten metal stirrer utilizing electromagnetic forces.

A still further object of this invention is the provision of a new andbetter electromagnetic molten metal stirrer wherein minimum heating ofthe stirrer is achieved with maximum utilization of the polyphase powersupplied.

A further object of this invention is the provision of a new and bettermolten metal stirrer capable of achieving stirring forces having asignificant vertical component, which component is reversible.

Yet another object of the present invention is to provide a novel vacuumdegassing and molten metal stirrer.

Other objects will appear hereinafter.

For the purpose of illustrating the invention, there are shown in thedrawings forms which are presently preferred; it being understood,however, that this invention is not limited to the precise arrangementand instrumentalities shown.

FIGURE 1 is a cross sectional View of a vacuum degassing furnaceutilizing the molten metal stirrer of the present invention.

FIGURE 2 is a crosssectional view of a second type of molten metalstirrer utilizing the principles of the present invention used to stirmolten metal in a teeming ladle.

Y FIGURE 3 is a phase time diagram of the four phase electrical powersystem of FIGURES 1 and 2.

In FIGURE 1, there is shown apparatus incorporating the principles ofthe present invention.

The apparatus 10 includes a vacuum chamber 12 having a cup-shaped.bottom portion 14 and a cover or lid 16. The cover 16 has a peripheralflange 18 cooperating with a peripheral flange 20 on the upper edges ofthe bottom portion 14 to form a vacuum seal. The vacuum chamber 12 has asteel outer lining 22 and a refractory inner liner 24. A vacuum pump 26has its inlet conduit 28 connected to a suitable vacuum conduit 30 atthe base of the bottom portion 14.

Within cup-shaped bottom portion 14 there is provided a suitable metalladle receiving stand 32 having a ladle recess 34 for receiving asuitable ladle 36. The ladle 36 has a steel outer shell 38 and arefractory inner liner 40. Molten metal 42 is placed in the ladle 36 fordegassing purposes. The ladle 36 is supported on the stand 32. bysuitable trunnions 44 and 46 which rest on the upper surface of thestand 32. The apparatus hereinabove described is for degassing moltenmetal. That is, the vacuum pump 26 withdraws the atmosphere from thechamber 12 so as to surface degas the molten metal 42 in the ladle 36,In order to better aid the area degassing, electromagnetic stirring hasheretofore been provided by the provision of coils mounted on the stand32 which were energized by alternating current. However, such apparatuswas inefficient in that the steel shell 38 would absorb theelectromagnetic energy prior to its passage into the molten metal thuslimiting the efliciency of the apparatus.

For this reason, the magnetic stirrer 48 of the present invention hasbeen provided. The magnetic stirrer 48 is mounted on the inner surface24 of the lid 16 by a plurality of supports 50. The magnetic stirrer 48is fingershaped and comprises a replaceable outer cup-shaped shell 52.The replaceable cup-shaped shell 52 acts as a protective sleeve for themagnetic stirrer 48 and must be manufactured of a material which isthermal shock resistant, a poor heat conductor, and a good electricalinsulator. One such material would be cast sintered silica. A graphitebased silica carbon would also be a desirable material for the sleeve.In view of the extremely high temperature conditions which theprotective sleeve or shell 52 must operate under, the shell should beinexpensive enough to be replaced after a few uses.

A preferred construction for the sleeve 52 comprises a ceramic sleevewith a ceramic plug at the bottom. This may be of the tar bonded typewhich will give it added strength and prevent cracks from developing,through which the molten metal could run.

The sleeve 52 may also be made of graphite with a closed end. Such asleeve could be machined in one solid piece and then covered with a washof alumina or similar material to prevent the graphite from beingattached by the molten metal. A conventional refractory may be insidethe graphite sleeve to thermally insulate the coil.

The shell 52 is cased about a suitable protective liner 54 which furtheracts as a former for the inductive coils mounted within the finger 48.

Four coils 56, 58, 60 and 62 are mounted within the liner 54 concentricwith the vertical axis of the protective liner 54. The four coils 56,58, 60 and 62 are connected to a polyphase low frequency sourcegenerally designated by the numeral 64. The coils 56, 58, 60 and 62 areso connected by their passage through the lid 16 to bushings (not shown)mounted on the outer surface 22 of the cover 16. The low frequencysource 64 is denoted schematically in FIGURE 1. By a low frequencysource is meant a source operating within the frequency range of .1cycle to 60 cycles per second. At the higher range of these he quencies,heating is effected of the molten metal. At the lower frequencies,maximum force is obtained for better stirring. However, it should beunderstood that the frequency utilized by the source 64 will be withinthe current, voltage and power ranges suitable for the practicalstirring application.

However, in designing a polyphase stirring system to 3 7 meet therequirements discussed previously there are conditions to be met.Initially, the frequency of the alternating magnetic field achieved bythe coils 56, 58, 60' and 62 must be chosen so that the depth of currentpeneration will be less than the thickness of the molten metal bath tobe stirred. The depth of current penetration d is defined by the wellknown relationship d (inches) =1.985\/L where 1- is the resistivity ofthe molten metal 42 in microhm centimeters, is the magnetic permeabilityof the molten metal 42 and f is the frequency of the polyphase lowfrequency source 64. The factor 1' is of the order of two hundredmicrohm centimeters for molten steel and ,u is essentially unity formolten metals. The dimensional value of current penetration d representscurrent peneration into the thickness of the molten metal 42 surroundingthe shell 52. That is, the thickness of the metal 42 is equal to theinside radius of refractory liner 40 minus the outside radius ofrefractory shell 52. The ratio of d to the radius of the inner surfaceof the ladle should vary bewhere K is a ratio, d is the depth ofpeneration for the liner 5,4 material, r is the mean radius of the liner54, and i is the thickness of the liner 54. It has been found by teststhat a value of approximately 10 for K in Equation 2 is a reasaonableand workable balance between the heat produced in the liner which isconsidered wasted, and the stirring effects produced in the molten metalas discussed above. The value of K greater than 10 is advantageous and avalue as low as 5 may be tolerable.

Thus, Equation 2 can be rewritten as follows:

d 2 t (inches) or preferably 0.47, t, Hm (inches) (4) where 'r is theresistivity of the liner 54 in microhm centimeters, and ,u is themagnetic permeability of the steel of the liner 54.

Under the conditions of Equation 4 the energy wasted in the liner 54might be half the power inducedin the molten metal 42, but suchconditions are operative.

The proportions of the inducing coils 56, 58, 60 and 62 likewise requireproper selection. Aside from providing the proper number of turns,insulation, etc., adequate for the input conditions, it'is advantageousto arrive at a means for determining the number of coils to be usedtogether with the number of phases and the order of connection. Certainconsiderations would normally indicate that a large number of coils andphases is desired. However, in practice, it is more economical to usecoils appropriate to two phase or three phase supply systems. It is anadvantage under certain conditions to use the normal complementary fourphase or six phase system so that the inducing coils as a complete unitgenerate no sig nificant currents in the metallic structures thatenclose the stirring assembly. For example, it is desirable to preventthe generation of any significant currents in the steel shell 22 of thevacuum chamber 12. It is herein under-' stood that the four phase systemmeans that there is one mangetically reversed coil for each directlyconnected phase coil for a two phase supply and a six phase system coilprogression. A complete set of coils may add up to 7 an integralmultiple of three hundred and sixty electrical degrees. The distancerepresenting three hundred and sixty electrical degrees or 1 cycleindicates the linear phase velocity. For example, if the frequency were1 cycle per second and the coils 56, 58, 6t) and 62 necessary to make upthe three hundred and sixty degrees were spaced over a one footdistance, then the phase velocity would be one foot per second.

Although complete sets of coils adding up to integral multiples of threehundred and sixty electrical degrees are desired, a three phase systemwith phase coils spaced sixty electrical degrees apart could be used.The sixty degrees spacing would be achieved by the utilization of areversed middle coil. This three phase system would add up to onlyelectrical degrees and thus would induce an instantaneous magnetic fieldwhich might elfect surrounding structures or cause undesired heating ofthe molten metal in the ladle, but would still be elfective to achievethe desired stirring.

It has been found that for producing linear motion of the molten metal42 in the ladle 36, relatively short phase coils are most effective.Analysis of this result indicates that adjacent phase coils, short inaxial length relative to their diameter, have an increased degree ofmutual inductance. The magnetic field mutually shared by successivecoils is a measure of the translational energy appearing as aunidirectional force on the molten metal 42. This is certainlydesirable. Hence it has been found that coils should be no longeraxially than about the radius thereof and may decrease to lower lengthswithin practical limits.

If the phase coils become too short (the axial length thereofdecreaseslwithout increasing the number of electrical phases, therecomes a point where the complementary or reversed phase coil reduces thenet flux intercepting the molten metal to an impractical value. The netflux intercepting the molten metal then only exists as shallow whorlsprogressing along the outer surface of the cup-shaped shell 52 of themagnetic stirrer 48 in a systematic manner, but without sufficientcoupling within the molten metal 42. 7

Thus, spacing between oppositely polarized coils of the same phaseshould, therefore, be such that the distance between the axial centersof the said polarized coils (56, 60 or 58, 62), should not be less thantwice the sum of the radial distance between the outside of the coilsand the outside of the shell 52 plus one-fifth the depth of currentpenetration d into the molten metal as per Equation 1. For assembliesusing the same number of coils as the electrical phase number, the axialcoil length may range between the value of the radius of the coil downto a value equal to the radial distance between the coil and the moltenmetal. In this last assembly, it is assumed that the coils are connectedfor consistent progression in electrical degrees but there are nooppositely polarized coils.

However, as stated previously, the preferred embodiment ofthe presentinvention utilizes oppositely polarized coils so as to prevent thegeneration of significant currents in the metallic structures that mightenclose the stirring assembly. 7

In operation, the lid 16 supporting stirrer48 is placed on bottomportion 14 in vacuum sealing relation. The vacuum chamber 12 isevacuated by pump 26 and stirrer 48 is energized. As the molten metal 42is stirred gas entrained therein rises to the surface and is drawn offby the pump 26. The stirring action increases the rate at which themolten metal 42 may be degassed, and also increases the effectiveness ofthe degassing.

In FIGURE 2, there is shown a second embodiment of the present inventionin which a magnetic stirrer 48 is utilized with a teeming ladle 36 tostir molten metal 42'. The teeming ladle 36' rests on a suitable ladlereceiving stand 32'. The ladle 36 is not in a vacuum chamber, but isstill desirable to stir the molten metal 42 for reasons such as theadding of alloying materials or the like. The molten metal stirrer 48'is carried by a hook 66 connected to an overhead crane in a steel mill.Thus the magnetic stirrer 48 can be moved about a steel mill to whereverit is needed. The hook 66 is connected to suitable cables 68 secured tothe outer edges of a spider 70 spanning the top of the molten metalstirrer 48'. The stirrer 48 has a replaceable outer cup-shaped shell 52'and an inner protective liner 54 similar to the shell 52 and liner 54 ofFIGURE 1. Coils 56', 58, 60' and 62 are mounted within the liner 54 andconnected to a suitable source of polyphase low frequency power in themanner of coils 56, 58, 60 and 62 of FIGURE 1. Additionally, the spider70 supports a ferromagnetic core 72 which extends axially through thecoils 56', 58, 60' and 62'. The core 72 improves the efficiency of themagnetic stirrer by substantially increasing the flux generated by thepolyphase power supply.

The design of the coils 56', 58', 60' and 62' and the protective liner54' fall wtihin the design criteria discussed with respect to theembodiment of FIGURE 1. In both embodiments, all coils have beenconsidered to be substantially the same axial length, but it will beunderstood that they could vary diametrically to accommodate a taper ofthe magnetic stirrer 48 or 48' or a taper of the ladles 36 and 36'.

Thus, better molten metal stirring apparatus has been achieved which canbe easily transported to any part of a steel mill. Additionally, thecoils could be water cooled by providing suitable Water conduits to themagnetic stirrer. The molten metal stirrer of the present inventionachieves better stirring by utilizing a polyphase supply whicheliminates extraneous magnetic fluxes and mixes the molten metal in onedirection uniformly from top to bottom of the ladle. By merely reversingthe energization of the phase coils, the direction of mixing can bereversed. Further, heating of the protective liner 54 has been minimizedby designing the thickness of the liner in accordance with predeterminedspecifications.

Further, efficient operation of the polyphase mixing has been enhancedby setting ratio of the depth of current penetration in the molten metalto the radius of the inside surface of the ladle betwen .3 and .9 andpreferably between .5 and .8. Oppositely polarized coils have beenspaced from each other a distance not less than twice the sum of theradial distance between the outside of the coil and the outside of therefractory liner of the magnetic stirrer was one-fifth the depth ofcurrent penetration into the molten metal. Still further, the coils havebeen designed with an axial length equal to or less than the radius ofthe coils.

Thus, molten metal mixing has been achieved in an economical andpractical manner by staying within the guidelines set forth.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification as indicating the scope of theinvention.

We claim:

1. A magnetic stirrer comprising a plurality of spaced electrical coilsmounted within a protective sleeve, said sleeve enveloping said coils sothat matter to be stirred cannot flow through coils, said sleeve havinga longitudinal axis, mounting means, said mounting means mounting saidsleeve with its longitudinal axis disposed vertically, said stirrerhaving a circular cross section, said spaced electrical coils beingmounted within said sleeve concentric with and spaced along said sleevelongitudinal axis, said protective sleeve being cup-shaped, the outersurface of said cup-shaped sleeve being made of a thermal shockresistant heat and electric insulating material, said sleeve including aliner on the inner surface thereof, said liner acting as a former forsaid coils, said liner having a thickness less than d /5r in incheswhere d is the depth of current penetration for the liner material and ris the mean radius of the liner.

2. The magnetic stirrer of claim 1 wherein the thickness of the liner isequal to or less than oA-T /l-b l' f in inches where T is theresistivity of the liner in microhm centimeters, ,u is the magneticpermeability of the liner material and f is the frequency of thepolyphase current in the coils.

3. A magnetic stirrer comprising a plurality of spaced electrical coilsmounted within a protective sleeve, said sleeve enveloping said coils sothat matter cannot flow through the coils, said sleeve having alongitudinal axis, mounting means, said mounting means mounting saidsleeve with its longitudinal axis disposed vertically, said stirrerhaving a circular horizontal cross section, said spaced coils beingmounted within said sleeve concentric with and spaced along said sleevelongitudinal axis, said electrical coils including two sets of coils,each coil in one set of coils being provided with a respectivemagnetically reversed coil in the second set of coils, associated coilsin said first and second set being spaced axially one from another withat least one non-associated phase coil being placed between each pair ofassociated coils in consistent phase progression, said coils adding upto integral multiples of one cycle of the applied electric power.

4. The magnetic stirrer of claim 3 wherein each of said coils has anaxial length less than the radius of the coil.

5. The magnetic st-irrer of claim 4 wherein the distance between axialcenters of associated coils is equal to or greater than twice the sum ofthe radial distance between the outside of the coils and the outsidesurface of the protective sleeve plus one-fifth the depth of currentpenetration int-o molten metal to be stirred, the depth of currentpenetration in inches being equal to where 1- is the resistivity of themolten metal in microhm centimeters, and ,u. is the magneticpermeability of the molten metal.

6. A magnetic stirrer comprising a plurality of spaced electrical coilsmounted within a protective sleeve, said sleeve enveloping said coils sothat matter cannot flow through said coils, said sleeve having alongitudinal axis, mounting means, said mounting means mounting saidsleeve with its longitudinal axis disposed vertically, said stirrerhaving a circular horizontal cross section, said spaced electrical coilsbeing mounted within said sleeve concentric with and spaced along saidsleeve longitudinal axis, each of said coils having an axial coil lengthequal to a value between the coil radius and a value equal to the radialdistance between the coil and the outside surface of the protectivesleeve, the number of coils being equal to the phase number of anapplied low frequency polyphase current.

7. Apparatus comprising a container adapted to contain molten metal,said container having an opening therein, a magnetic stirrer extendinginto said container to stir molten metal in said container, saidmagnetic stirrer comprising a plurality of spaced electrical coils, aprotective sleeve, said protective sleeve enveloping said coils so thatmolten metal within said container cannot injure said coils, and a lowfrequency polyphase supply system supplying electrical power to saidcoils, said coils progressively varying in phase angles, means forsupporting said stirrer in said container spaced from the bottom of saidcontainer, said means supporting said stirrer with the longitudinal axisof said stirrer disposed vertically, said polyphase supply systemfrequency being chosen so that the depth of current penetration ofmolten metal between the container and the sleeve is less than thethickness of the molten metal to be stirred, said depth of currentpenetration being equal to a (inches) 1.9s I af where 1- is theresisitivity of the molten metal in microhm centimeters, ,u. is themagnetic permeability of the molten metal, and f is the frequency of thepolyphase supply system.

8.The molten metal stirring apparatus of claim 7 wherein the ratio ofthe depth of current penetration of the molten metal to the radius ofthe inner surface of the container varies between the limits of .3 and.9.

9. Molten metal stirring apparatus of claim 8 wherein the ratio of thedepth of current penetration of the molten metal to the radius of theinner surface of the container varies between the limits of .5 and .8.

10. Apparatus in accordance with claim 7 wherein said container ismounted in a vacuum chamber, said chamber including vacuum pump means toevacuate said chamber.

11. Apparatus in accordance with claim 7 wherein said supporting meanscomprises a vacuum chamber enclosing said container and stirringapparatus, and pump means for evacuating said vacuum chamber.

12. Apparatus comprising a container adapted to con 8 tain molten metal,a magnetic stirrer extending into said container to stir molten metal insaid container, said magnetic stirrer comprising a plurality of co-axialspaced electrical coils, a protective sleeve, said spaced electricalcoils being mounted within said sleeve concentric with and spaced alongthe sleeve longitudinal axis, said sleeve having a circular transversecross section and being closed at one end thereof, a protective linermounted within said sleeve co-axial of said longitudinal axis, saidprotective liner forming said coils, and a low frequency polyphasesupply system supplying electrical power to said coils, said coilsprogressively varying in electrical phase angle.

13. Apparatus in accordance with claim 12 wherein said sleeve is made ofa thermal shock resistant heat and electrical insulating ceramicmaterial.

14. Apparatus in accordance with claim 13 wherein a ceramic plug ismounted in the closed end of said sleeve.

15. Apparatus in accordance with claim 13 wherein said sleeve is made ofa graphite material with an inner sleeve of refractory material.

References Cited UNITED STATES PATENTS 2,826,666 3/1958 Cater 219l0.65 X

2,972,652 2/1961 Seemann et al. 1331 X 3,053,921 9/1962 Tagliaferri 13283,251,921 5/1966 Hartley 1 328 FOREIGN PATENTS 1,269,156 7/ 1961 France.

RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner.

12. APPARATUS COMPRISING A CONTAINER ADAPTED TO CONTAIN MOLTEN METAL,MAGNETIC STIRRER EXTENDING INTO SAID CONTAINER TO STIR MOLTEN METAL INSAID CONTAINER, SAID MAGNETIC STIRRER COMPRISING A PLURALITY OF ACO-AXIAL SPACED ELECTRICAL COILS, A PROTECTIVE SLEEVE, SAID SPACEDELECTRICAL COILS BEING MOUNTED WITHIN SAID SLEEVE CONCENTRIC WITH ANDSPACED ALONG THE SLEEVE LONGITUDINAL AXIS, SAID SLEEVE HAVING A CIRCULARTRANSVERSE CROSS SECTION AND BEING CLOSED AT ONE END THEREOF, APROTECTIVE LINER MOUNTED WITHIN SAID SLEEVE CO-AXIAL OF SAIDLONGITUDINAL AXIS, SAID PROTECTIVE LINER FORMING SAID COILS, AND A LOWFREQUENCY POLYPHASE SUPPLY SYSTEM SUPPLYING ELECTRICAL POWER TO SAIDCOILS, SAID COILS PROGRESSIVELY VARYING IN ELECTRICAL PHASE ANGLE.